1. Field of the Invention
The present invention relates to heat exchange reactors and methods of constructing heat exchange reactors.
2. Discussion of the Background
Heat exchange reactors are often employed to carry out chemical reactions where significant quantities of heat must be added or removed from a first reacting fluid to a second heat transfer fluid, which may or may not be reacting. These heat exchange reactors often bear a strong resemblance to simple heat exchangers, but are often provided with additional features such as fixed beds of catalysts, specialized flowpath designs, exotic materials and the like. Heat exchange reactors are constructed in many forms, including plate-fin and tubular arrays.
An example of a reaction conducted in heat exchange reactors is the steam reformation of hydrocarbon feedstocks to produce hydrogen-containing gas mixtures. In this process, a mixture of steam and hydrocarbon is passed through one fluid circuit while a hot fluid, usually combustion product, flows through a separate fluid circuit and transfers heat into the reacting first fluid to promote the highly endothermic steam reforming reaction. An example of a plate-fin type hydrocarbon steam reformer is shown in U.S. Pat. No. 5,733,347 to Lesieur. Several examples of tubular heat exchange reformers have been revealed, for example U.S. Pat. No. 3,446,594 to Buswell et al. An advanced tubular reformer configuration which offers significant advantages over other configurations is disclosed in U.S. application Ser. Nos. 09/642,008 and 09/928,437 to Lomax et al., which are incorporated herein by reference in their entirety.
The present inventors have determined that many heat exchange reactors face a serious mechanical design challenge due to the temperature differences between the reacting first fluid and the second heat transfer fluid. These temperature differences set up thermal strains, or displacements, due to differential expansion of the material of construction of the heat exchange reactor. If free expansion is not allowed for in the configuration of the heat exchange reactor, then the unrealized strains result in thermal stresses. The thermal stresses are particularly challenging in hydrocarbon steam reformers because the temperature gradients are generally very high. Further, modem heat exchange reactors for steam reforming strive to reduce the physical size of the reactor to reduce cost and facilitate their employment in space and weight sensitive applications such as vehicles. The reduction in physical size results in an aggravation of the problem of thermal stresses by drastically-increasing the thermal gradients in the heat exchange reactor.
In tubular heat exchange reactors in general, and in the improved reactor of U.S. application Ser. No. 09/642,008 in particular, one route to achieving a more compact reactor is the provision of baffle features to induce flow of the second fluid in a direction substantially normal or perpendicular to the axis of the tubes. Such a flow arrangement is termed cross flow. By placing several baffle features along the length of the heat exchange reactor tubes, the second heat exchange fluid may be induced to flow across the tube array several times. Through optimal selection of the number and spacing of baffles, the mechanical configuration of a tubular heat exchange reactor may be optimized for factors such as physical size, second fluid pressure drop, and other important features.
The provision of features in tubular array heat exchange reactors presents formidable challenges due to the thermal gradients along the axis of the tubes. These challenges are due to the fact that thermal expansion at a given temperature is related to three factors including temperature, material of construction, and physical dimension. The thermal expansion is expressed by the relationship ΔL=αΔTLo, where α is nominally a constant determined by the material of construction (i.e., a coefficient of thermal expansion), ΔT is the variation between the temperature of interest and a reference temperature, and Lo is the initial length of the feature at the reference temperature. Because the baffles are generally planar parts of large extent normal to the tubes, the expansion of the baffles with temperature change is very large compared to the expansion of the tubes themselves, which are generally much smaller in a plane normal to the axis of the tubes.
In the advanced heat exchange steam reforming reactor of the type disclosed in U.S. application Ser. No. 09/642,008, the inventors have determined that the problems associated with thermal expansion are compounded because the array of tubes is colder at both ends than in the center. Since the tubes are joined to header plates of rigid pressure heads at both ends of the tubes, the relative expansion of the tubes in the plane normal to the longitudinal axis of the tubes is fixed by the temperature of the pressure heads. In the zone of higher temperature located in the center between the ends of the tubes, the planar parts normal to the tubes (e.g., baffles and planar fins) expand at a proportionally greater amount than the colder pressure heads. The inventors have determined that if the entire reactor is constructed of materials with similar coefficients of thermal expansion (α), then the planar features are apt to exert severe forces normal to the axis of the tubes. These forces can cause premature structural failure of the reactor unless exceptionally strong tubes are employed, which is undesirable for several reasons, including an objectionable increase in the material usage in the construction of the reactor, as well as an attendant increase in volume and weight of the reactor.